Evolutionary and Structural Perspectives of Plant Cyclic Nucleotide-Gated Cation Channels

Abstract

Ligand-gated cation channels are a frequent component of signaling cascades in eukaryotes. Eukaryotes contain numerous diverse gene families encoding ion channels, some of which are shared and some of which are unique to particular kingdoms. Among the many different types are cyclic nucleotide-gated channels (CNGCs). CNGCs are cation channels with varying degrees of ion conduction selectivity. They are implicated in numerous signaling pathways and permit diffusion of divalent and monovalent cations, including Ca2+ and K+. CNGCs are present in both plant and animal cells, typically in the plasma membrane; recent studies have also documented their presence in prokaryotes. All eukaryote CNGC polypeptides have a cyclic nucleotide-binding domain and a calmodulin binding domain as well as a six transmembrane/one pore tertiary structure. This review summarizes existing knowledge about the functional domains present in these cation-conducting channels, and considers the evidence indicating that plant and animal CNGCs evolved separately. Additionally, an amino acid motif that is only found in the phosphate binding cassette and hinge regions of plant CNGCs, and is present in all experimentally confirmed CNGCs but no other channels was identified. This CNGC-specific amino acid motif provides an additional diagnostic tool to identify plant CNGCs, and can increase confidence in the annotation of open reading frames in newly sequenced genomes as putative CNGCs. Conversely, the absence of the motif in some plant sequences currently identified as probable CNGCs may suggest that they are misannotated or protein fragments.

Multiple Calmodulin-binding Sites Positively and Negatively Regulate Arabidopsis CYCLIC NUCLEOTIDE-GATED CHANNEL12

“…Early studies with select isoforms led to the hypotheses that Ca 2+ /CaM provides allosteric negative regulation of plant CNGC function and that a single CaMBD is conserved within the aC-helix of every plant CNGC (Arazi et al, 2000;Hua et al, 2003;Köhler and Neuhaus, 2000;Li et al, 2005;Kaplan et al, 2007); however, discoverydriven investigations into how CaM interacts with and regulates various plant CNGC isoforms have been lacking. CNGC12 (specifically) had been previously suggested to not bind CaM due to low conservation at its aC-helix region (Zelman et al, 2012); however, our study demonstrates that this isoform possesses novel CaMBDs at both its cytosolic N and C termini, providing evidence that an individual plant CNGC isoform possesses multiple CaMBDs.…”

“…The region corresponding to the conventional aC CaMBD within the CNBD of CNGC12 is poorly conserved relative to other members of the CNGC family in Arabidopsis (Zelman et al, 2012). Thus, we empirically investigated the CaM binding properties of this region and searched for novel CaMBDs in CNGC12 (see Supplemental Figure 1 for delineation data).…”

“…Previous studies have hypothesized that the location of the CaMBD within the CNBD allows CaM to compete with cyclic nucleotide monophosphate as a ligand in the allosteric gating of channel conductance (Hua et al, 2003;Kaplan et al, 2007;Swarbreck et al, 2013). It was originally suggested that this site is the only CaMBD in plant CNGCs, while sequence conservation of this region varies considerably between CNGC isoforms (Zelman et al, 2012). Recently, Arabidopsis CNGC20 was found to bind CaM via a distinct isoleucine-glutamine (IQ) motif adjacent to but not overlapping the aC-helix (Fischer et al, 2013), suggesting that plant CNGCs, like mammalian CNGCs, may possess a variety of CaMBDs.…”

“…CNGCs are nonselective cation channels that are hypothesized to function as Ca 2+ channels in plants (Dietrich et al, 2010;Jammes et al, 2011;Zelman et al, 2012) and are characterized by conserved structural components, including a short cytosolic N terminus, six transmembrane helices (S1-S6) with a pore-forming region between S5 and S6, and a cytosolic C terminus containing a cyclic nucleotide binding domain (CNBD; Kaupp and Seifert, 2002;Matulef and Zagotta, 2003). The CNBD mediates channel gating by cyclic nucleotide monophosphates such as cAMP and/or cGMP (Mäser et al, 2001;Kaupp and Seifert, 2002;Matulef and Zagotta, 2003;Kaplan et al, 2007;Zelman et al, 2012), though studies of this mechanism are largely restricted to animal isoforms.…”

Phytosulfokine Regulates Growth in Arabidopsis through a Response Module at the Plasma Membrane That Includes CYCLIC NUCLEOTIDE-GATED CHANNEL17, H+-ATPase, and BAK1

“…PSKR1 interacts with BAK1 and with the H + -ATPases AHA1 and AHA2. BAK1, AHA1, and AHA2 in turn bind to the cyclic nucleotide-gated cation channel CNGC17 that is subject to regulation by cGMP and CaM (Zelman et al, 2012;Fischer et al, 2013). AHAs, CNGC17, and the promiscuous coreceptor BAK1 are predicted to form a functional core module that links proton extrusion to cation uptake across the plasma membrane.…”